For future printing based on multiple patterning and directed self-assembly, critical dimension and overlay requirements
become tighter for immersion lithography. Thermal impact of exposure to both the projection lens and reticle expansion
becomes the dominant factor for high volume production. A new procedure to tune the thermal control function is
needed to maintain the tool conditions to obtain high productivity and accuracy. Additionally, new functions of both
hardware and software are used to improve the imaging performance even during exposure with high-dose conditions.
In this paper, we describe the procedure to tune the thermal control parameters which indicate the response of projection
lens aberration and reticle expansion separately. As new functionalities to control the thermal lens aberration, wavefront-based
lens control software and reticle bending hardware are introduced. By applying these functions, thermal focus
control can be improved drastically. Further, the capability of prediction of reticle expansion is discussed, including
experimental data from overlay exposure and aerial image sensor results.
Accurate overlay with high throughput is the key to success in multiple-patterning lithography. To achieve accurate overlay, the imaging system must control and minimize the thermal aberration and distortion. There are several sources of thermal aberration in an immersion lithography system: (1) reticle deformation by reticle heating; (2) air temperature fluctuation near the reticle; (3) thermal aberrations from the projection lens; and (4) immersion water temperature fluctuation. All aberrations and distortion are impacted by these sources and need to be minimized for accurate overlay. In this paper, we introduce our approach and technologies for the control of thermal aberrations.
In the history of DUV (Deep Ultra Violet) microlithographic lens design, three kinds of leaps have occurred to maintain
the progress of technology in the semiconductor industry. The first step is the application of aspherical elements. This
allowed us to increase NA up to around 0.9. The second innovation is water immersion. Thanks to the 1.44 refractive
index of water, and because the numerical aperture (NA) is defined as the product of the sine of the maximum ray angle
on the image plane and the refractive index in the image space, even with a lower maximum ray angle on the imaging
plane than dry with a lens, we can achieve NA of 1.07. The latest technological jump is the development of
catadioptric lens systems, which are roughly defined as the combined usage of refractive element(s) and reflective
element(s). The catadioptric system allows us to achieve a full field 1.3NA projection lens that is used in our scanner
NSR-S610C. In this paper we discuss optical design concepts and some challenges for catadioptric lenses. In addition,
current lens performance including wavefront, lens flare, and image vibration are shown.